1. Field of the Invention
This invention relates generally to the execution of decimal arithmetic instructions by a commerical instruction processor of a data processing system, and more specifically to apparatus which, during the execution of a decimal divide instruction by the successive subtraction method, tests the high order positions of the divisor and the remainder to anticipate a negative result of the next subtraction.
2. Description of the Prior Art
Data processing systems commonly perform the decimal divide instruction by successive subtractions of the dividend by the divisor. The subtractions are carried out by aligning the most significant decimal digit positions of the dividend and divisor and subtracting a decimal digit at a time, least significant decimal digit position first. The high order quotient decimal digit is generated by counting the number of subtractions of the dividend by the divisor that gives a positive remainder. A negative subtraction result indicates that no further subtractions are needed, the divisor is added back to the remainder, and the unsuccessful subtraction is not counted.
The divisor is shifted over one digit position to the right and successive subtractions are made of the remainder to generate the next lower order quotient decimal digit. Such a divide technique is described in "Digital Computer Design Fundamentals" by Y. Chu, published by McGraw-Hill in 1962.
It was desirable to eliminate the last subtraction and add back steps to reduce the time period for executing the decimal divide instruction. This was accomplished by comparing the high order decimal digits after a subtraction to anticipate the negative result of the next subtraction. However, this test required a number of steps which reduced the throughput.
It should be understood that the references cited herein are those of which the applicants are aware and are presented to acquaint the reader with the level of skill in the art and may not be the closest reference to the invention. No representation is made that any search has been conducted by the applicants.